I'm no mol bio person but why is the uncut control (2nd lane) running as two bands so far apart? Isn't plasmid supposed to run in three bands pretty close to each other? Student A (lane 3) looks as if he has (i) Forgot to add enzyme/added too less.(ii) Added way too much plasmid.The later seems more likely as the band that you have drawn is very thick compared to others. Enzyme activity is measure in ug of DNA that it can cleave; if concentration of your template is beyond the capacity of the enzyme that you have added then you will not get an expected fragmentation pattern.